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308 Analytical Techniques for Atmospheric Measurement

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Chapter 7

Chemical Methods: Chemiluminescence,

Chemical Ampliﬁcation,

Electrochemistry, and Derivatization

Andrew J. Weinheimer

7.1 Introduction

7.1.1 Scope

This chapter covers techniques that require chemical conversion of the target species for

a measurement to take place. Foremost among these is chemiluminescence, which is the

phenomenon of light production by a chemical reaction. Whether the fluorescent species

is one of the initial reaction products or one produced after energy transfer to another

species, this phenomenon enables the measurement of many species of atmospheric

interest. It is a fundamental process in many of the measurement techniques described

in this chapter. In some cases the chemiluminescence mechanism is understood in detail,

in others it is not.

The chapter will cover the measurement of O

by its chemiluminescent reaction with

a number of dyes, either in a liquid form or adsorbed onto solid substrates. These

are inexpensive, low-power, light-weight techniques that require careful and frequent

calibration for quantitative results. When the experimental situation allows, use is made

of heavier, more stable, and more accurate instruments based on the gas-phase chemilu-

minescent reaction of O

with either ethene or NO. Conversely nitric oxide is normally

detected by its chemiluminescent reaction with O

, and this technique allows other

nitrogen compounds to be converted to NO for subsequent measurement, provided the

conversion is good – a paramount concern for this class of measurements.

Like O

,NO

can be measured by its chemiluminescent reaction with a dye, either as

liquid or adsorbed onto solid substrates, and this technique is used not only for measuring

itself but also for the measurement of peroxy radicals by measuring the amplified

signal resulting from the conversion of a single peroxy radical to a larger number of NO

molecules by a chemical amplifier technique. In a related technique, a peroxy radical is

converted to a larger number of H

molecules, which are detected using the CIMS

technique (Chapter 5).

Also included are several techniques involving the ‘scrubbing’ of soluble gases into

solution for detection by liquid-based techniques, generally after conversion into other

species and sometimes involving chemiluminescence or fluorescence detection as the final

312 Analytical Techniques for Atmospheric Measurement

step in the measurement process. We also include an electrochemical technique for O

upon which much of our knowledge of atmospheric ozone is based.

The order of coverage within the chapter goes somewhat chronologically, but with

some grouping according to classes of species.

7.1.2 Measurement highlights

Some measurements resulting from this class of instruments will be highlighted in the

course of describing the respective instruments. These measurements span the spectrum

from the routine monitoring of pollutants such as NO

on the ground in urban environ-

ments to specialized high-altitude aircraft studies of topics such as stratospheric ozone

depletion. These instruments have been essential to providing an accurate picture of

pollution in the urban environment as well as providing insight into the processes that

control ozone throughout the atmosphere, whether it relates to how anthropogenic

emissions affect stratospheric O

or how natural processes such as lightning can foster

production in the troposphere. The light weight of chemiluminescence detectors has

allowed them to be carried to great heights by balloons and rockets to sample regions

of the atmosphere not accessible to aircraft sampling. The use of electrochemical sondes

for measuring altitude profiles of O

from balloons has enabled the determination of

temporal trends and spatial distributions of atmospheric O

. The fast response of chemi-

luminescence detectors allows for eddy flux measurements of pollutants such as O

as well

as natural emissions such as isoprene from trees. The fast response also makes possible

the detection of fine-scale filamentary structures and thereby sheds light on atmospheric

mixing processes. It also allows the detection of fine-scale pollution plumes from power

plants and aircraft exhaust, thereby enabling our understanding of the processes that

occur there. The high sensitivity of chemiluminescence O

detectors enables meaningful

measurements in near-surface O

depletion events where a low detection limit is required.

This class of instruments also includes measurements of species central to the radical

chemistry of the atmosphere, both through the chemical amplifier measurements of

peroxy radicals themselves, HO

and RO

, and the liquid-based techniques used for

the measurement of reactive intermediates or radical reservoir species, such as H

OOH, HCHO, and HONO, whose measurement and modeling are essential to the

understanding of the radical chemistry in the atmosphere.

7.2 O

via heterogeneous chemiluminescence

The chemiluminescent reactions of O

with a variety of dyes have been used in instruments

that are generally portable and lightweight, and so have found application on balloon,

rocket, and aircraft platforms. Their rapid response and high sensitivity have also made

them suitable for flux measurements via eddy correlation. Some have even been developed

commercially. This class of instruments was initiated by the work of Bernanose & René

(1959) and Bowman & Alexander (1966), who made laboratory observations of the strong

chemiluminescence observed when a large number of organic compounds were exposed

to O

and made the suggestion that O

could be thus measured. For these reactions, the

Chemical Methods 313

mechanism for light production is generally poorly known, but it has been suggested

to involve the production of an excited product that transfers energy to the dye which

then fluoresces, as the chemiluminescence wavelengths are generally near the respective

fluorescent wavelengths.

7.2.1 Early years (luminol and rhodamine-B)

The first practical chemiluminescence instrument for atmospheric measurements was

developed by Regener (1960), who adsorbed luminol onto silica gel to produce a solid-

phase reagent for use in a balloon instrument that could go to high altitudes >30 km

where a liquid-based instrument would not work due to the low pressures and temper-

atures. A chemiluminescent disk was viewed by a photomultiplier tube across a small

gap, through which air was drawn for sampling, generating a signal proportional to the

partial pressure of O

. This type of arrangement is illustrated in Figure 7.1 taken from

a later work (Stevens & O’Keeffe, 1970), which illustrates common features of this class

of devices. Air is pumped or otherwise forced through a small gap (∼1cm) between

a chemiluminescent disk, or pool of liquid, and the window of a photomultiplier tube

(PMT) which views the chemiluminescence occurring at the surface of the reagent, or

also within the volume, in the case of a liquid. The small sample volumes facilitate rapid

response.

Citing an effect of moisture on the response of the luminol instrument, Regener (1964)

later adsorbed rhodamine-B on silica gel. Since moisture was found to gradually degrade

the response, a moisture trap was inserted upstream of the detection cell. In addition,

light also reduces the sensitivity, and O

consumes the reagent, so periodic calibrations

are needed to monitor the gradual degradation of the detector. The instrument appeared

to be specific to O

, and no significant interferences from NO

and SO

were found.

Such instruments were flown on hundreds of balloons in a sounding network as well as

on a jet aircraft. The balloon instrument weighed 1.0 kg and consumed very little power

(the luminol version used 0.4 W).

Hodgeson et al. (1970), working from one of Regener’s prototypes, developed an

improved version of the rhodamine-B instrument and described its operation in greater

detail than done in prior publications. Hodgeson et al. (1970) found aspects of the

prototype to be cumbersome, including the complexity of the recipe for preparation of

the chemiluminescent surface. They worked on improvements and published results of a

number of tests on the new version. They employed a small cylindrical reaction chamber

closely coupled to a two-inch photomultiplier tube, with the disk less than 13 mm from

the red-sensitive photocathode (the rhodamine-B emission peaks near 580 nm). This

small volume made possible a time constant for the detection of step changes in O

only a few seconds.

Hodgeson et al. (1970) determined that the most serious problem with the Regener

technique was the effect of water vapor on the rhodamine-B silica gel surface, which they

suggested was likely due to H

O adsorption and quenching of the excited state prior to

fluorescence. They substantially eliminated this effect by adding a hydrophobic agent,

silicone resin, to the silica gel surface. The sensitivity was then independent of water

vapor, as demonstrated in a test at 80% relative humidity, where the resin-treated surface

314 Analytical Techniques for Atmospheric Measurement

Figure 7.1 Diagram of a typical heterogeneous O

chemiluminescence instrument. Sample air is pumped

across a chemiluminescent disk which is viewed by a photomultiplier tube across a small gap ∼1–2cm.

In this case the sample air is diluted before detection, and provision is made for O

generation for

calibration purposes. (Figure 1 of Stevens & O’Keeffe, 1970, printed with permission from American

Chemical Society.)

maintained a constant sensitivity, while an untreated surface suffered a 50% decrease

over 30 min.

Potential interferents were tested. No interference was found for NO

,SO

, PAN,

, peracetic acid, H

S, propane, NH

, NO, and Cl

. In tests with variable flow rates,

it was found that once the flow was above a minimum rate, there was no dependence on

flow rate. At lower flows, a significant fraction of the O

is apparently consumed and so

the response is reduced, while at higher flows the consumed fraction is negligible, so the

response is independent of flow.

Chemical Methods 315

The instrument had a detection limit of better than 1 ppbv and was linear up to

400 ppbv. Its primary limitation was the gradual decay of sensitivity over time, so periodic

calibration was required. The calibration source was an Hg penray lamp operated under

controlled conditions, its output having been calibrated against a primary standard.

Hilsenrath and co-workers (Hilsenrath et al., 1969; Hilsenrath & Kirschner, 1980)

deployed a rhodamine-B detector on rocket-borne sondes to measure some 30 profiles

of ozone concentration, starting in the late 1960s. Flow through the instrument is driven

by the flow into an evacuated ballast volume that fills as the pressure equalizes during

descent on a parachute. With flows and pressures not as high as in other applications,

the rocket sonde operates in a low-flow regime where the signal is proportional to the

flow rate as well as the ozone concentration, so it is necessary to derive the flow from

measurements of pressure and temperature inside the ballast volume. With careful sensor

preparation and calibration processes, Hilsenrath and co-workers were able to achieve a

calibration stability of 5% or better, leading to measurements over the wide altitude range

of 20–65 km with an absolute accuracy of 12% and a precision of 6%. An altitude profile

measured with this instrument is shown as the solid line in Figure 7.2. The measurements

compare extremely well both with a rocket-borne optical technique (height-dependence

of downwelling UV flux) and with two in situ electrochemical sondes (Section 7.3),

except that the chemiluminescence technique shows a high bias at the lowest altitudes

<20 km, likely due to inaccuracies in the flow rate measurement and a residual glow

from the chemiluminescent disk.

16 September 1968 Rocket Chem.

16 September 1968 Rocket Optical

16 September 1968 ESSA & AFCRL Balloon

27 May 1960 Rocket Airglow

Ozone density (mol/cm

)

Altitude (kilometers)

Figure 7.2 O

proﬁles over the altitude range 15–70 km measured using different techniques: (a)

rhodamine-B chemiluminescent dye, (b) altitude proﬁle of downwelling UV ﬂux, (c) electrochemical

sondes, (d) rocket airglow. Techniques (a) and (c) are described in the text. (Figure 10 of Hilsenrath et al.,

1969, printed with permission from American Geophysical Union.)

316 Analytical Techniques for Atmospheric Measurement

7.2.2 Since mid-1980s (eosin Y, coumarin-47,

chromotropic acid)

Since the mid-1980s additional dyes have been used for the chemiluminescence detection

of O

, and two have found application in commercial instruments. The first of these is

eosin-Y, used as a solution in ethylene glycol. Ray et al. (1986) built a detection cell

in which a red-sensitive photomultiplier tube views a fiber pad across which the dye

solution flows continuously at 1 mL/min, as it is recirculated by a pump. Air is drawn

through the small gap between the PMT and the wetted fiber pad at flows of 1–9 L/min.

Hodgeson et al. (1970) had earlier tested eosin-Y adsorbed on silica gel and found

high sensitivity, but their surface showed a much more rapid long-term decay than for

rhodamine-B, making it unsuitable for long-term use. Hodgeson et al. (1970) report that

the chemiluminescence peaks near 560 nm.

Like rhodamine-B, eosin-Y detection is very fast and sensitive. At an air flow of 1 L/min

the response frequency is 1.5 Hz. Reduction of cell volume and use of an elevated air flow

of 9 L/min gave a 7 Hz response (Ray et al., 1986). In addition to slightly better sensitivity,

eosin-Y’s main advantage over rhodamine-B is that it does not suffer a water vapor

dependence. Drummond et al. (1991) describe a commercial, battery-powered eosin-Y

instrument with a response time of less than 0.5 s and a detection limit S/N = 3 of

1 ppbv for a 4 s measurement. Ray et al. (1986) found no interference from NO, NO

,or

PAN, while Drummond et al. (1991) found none from NO, NO

, HNO

,NH

or HCHO. As is typical with these instruments, calibration is achieved by periodically

supplying a known amount of O

to the inlet from a stable source calibrated with another

instrument.

A second commercial instrument employs coumarin-47 adsorbed on silica gel, as

described by Schurath, Güsten, and co-workers (Schurath et al., 1991; Güsten et al., 1992;

Güsten & Heinrich, 1996). An advantage of this dye is that its chemiluminescence peaks

in the 400–500 nm range, where blue-sensitive PMTs are available, and these are less

expensive than those required for the red-emitting dyes (rhodamine-B, eosin-Y). The

coumarin-47 instrument is also very fast and sensitive. The 90% response time is signifi-

cantly better than 0.1 s, with a 100 L/min flow, and the detection limit S/N =3 is better

than 50 pptv. A disadvantage is that its sensitivity increases with ambient H

O vapor,

and it is necessary to measure the H

O partial pressure and apply a laboratory-derived

sensitivity-enhancement correction factor. Attempts to eliminate the H

O dependence by

the use of hydrophobic or hygroscopic compounds, as done by Hodgeson et al. (1970)

for rhodamine-B, were unsuccessful. The O

sensitivity has a 22 s response time (90%)

to changes in water vapor, so it does not interfere with the measurement of rapid O

changes. There is a similar effect due to SO

. It was found that the silica gel disks freshly

impregnated with coumarin-47 needed to be sensitized by exposure to ozone (100 nbar

for 2 hr). Once this was done, the disks had a useful lifetime of about 50 hr, during which

the sensitivity is constant, but after which it decays rapidly. In a typical application, the

instrument needs calibration against an absolute instrument every half hour (Güsten &

Heinrich, 1996). No interferences were found for NO

, or PAN.

The shortcomings of the coumarin-47 instrument are balanced by its high sensitivity

and fast response, and by its ease of deployment because of its low weight (1.5 kg) and

low power consumption. It has found use in long-term ground-based measurements of

Chemical Methods 317

the vertical deposition flux of O

using eddy correlation (Güsten & Heinrich, 1996), as

well as use on a light aircraft to make measurements in complex, mountainous terrain,

where it would not be possible to fly larger, faster research aircraft (McKendry et al.

1998). In addition, the instrument has been used to study small-scale, filamentary features

of the ozone field generated by wave breaking in the lower stratosphere. For this it was

flown on a high-altitude aircraft (Egrett) together with a slow response (10–20 s) ozone

UV absorption instrument against which the much faster chemiluminescence instrument

could be continuously calibrated (Bradshaw et al., 2002).

Takayanagi et al. (2003) have developed a liquid-phase detector using chromotropic

acid. A novel feature of this technique is the need to photo-activate the chromotropic

acid by a few-second exposure to UV light just before it flows into view of the PMT.

This results in a stable, sensitive, and fast detector with a detection limit of 40 pptv and

10–90% and 90–10% rise/fall times of 130 and 80 ms. These are comparable to the speed

of the coumarin-47 technique but with flows of only 1.5 L/min as opposed to 100 L/min.

As with the other techniques, no interferences from NO

and H

were found, and, as

with other liquid techniques, it was not affected by relative humidity in the way that the

dry sensors are.

7.2.3 Summary

These heterogeneous techniques are appealing due to their high specificity for ozone,

low cost, low weight, and low power consumption, while having high sensitivity and

fast response. The fast response makes them useful for aircraft use and eddy corre-

lation measurements of O

fluxes. The dry techniques suffer from sensitivity changes

due to ambient water vapor, and all suffer from drift in the sensitivity, primarily as a

long-term decline in sensitivity as the reagent is consumed. Thus all require relatively

frequent calibration. Compared with the homogeneous chemiluminescence techniques

(Section 7.5), these heterogeneous techniques avoid the use of compressed, hazardous

gases (nitric oxide and ethene) used as reagents for the chemiluminescence reaction.

However, this is traded for the need to take great care in the preparation and/or

activation of the solid or liquid-phase chemiluminescent surfaces. In sum, the hetero-

geneous techniques find their use where weight and power are at a premium, such as

from balloons, aircraft, and rockets, and where speed is of utmost importance, as in

measurements of fluxes and in measurements from aircraft, but great care in calibration

must be exercised.

7.3 O

via electrochemistry

profiles up to 35 km altitude have been routinely measured by a network of several

dozen stations spread over the globe, with records going back to the 1960s and 1970s

for many of the stations. Two types of sondes have been used for the bulk of these

measurements and both are based on the reaction of dissolved potassium iodide (KI)

with O

in air as it is bubbled through the solution during balloon ascent:

2KI +O

O → I

+2KOH